There are many isolated endorheic lakes in the transvolcanic belt of Mexico, which are the result of volcanism or extreme flooding. Organisms living in the lakes have had ample time to differentiate, and endemism is well known and documented, especially among the fishes. Other organisms also show endemism, including salamanders, crayfish, and even birds and mammals. The same is true for the garter snake, Thamnophis eques (Reuss). Seven new subspecies are described in this paper, each from a different lake or from the remnants of a former large lake. Based on morphological differences in coloration and pattern, they are Thamnophis eques cuitzeoensis from El Lago de Cuitzeo, Thamnophis eques patzcuaroensis from El Lago de Pátzcuaro, Thamnophis eques insperatus from La Laguna de Zacapu, Thamnophis eques obscurus from El Lago de Chapala, Thamnophis eques diluvialis from Las Lagunas Atotonilco and Cajititlán and several isolated localities, Thamnophis eques scotti from El Lago de Magdalena, and Thamnophis eques carmenensis from La Lagunilla del Carmen. Among six of these, series of specimens were collected and studied in detail. The seventh (insperatus) is known only from a single imperfect individual.